This invention relates to a communication system utilizing a spread spectrum communication and, more particularly, to a communication system of this type capable of preventing a spread spectrum signal from being impaired due to variation in a transmission frequency or local oscillation frequency in a receiving section and thereby capable of accurately demodulating a received signal.
The spread spectrum communication is a communication system in which data is transmitted with a broadened band (i.e., with spread spectrum) by modulating data to be transmitted with a code system called a pseude noise signal. This communication system has the feature that data can be transmitted without being affected by a signal distortion or an interfering wave in the transmission path.
The basic principle of a direct spread system which is representative of the spread spectrum communication system will be described with reference to FIG. 2. In a transmitter 10, a spread code (i.e., pseudo noise signal) which repeats a predetermined pattern is generated by a PN system generator 12. Data signal for transmission is modulated with this spread code in a modulator 14 and a spread spectrum signal thereby is produced. A carrier (frequency fo) is modulated with this spread spectrum signal in a modulator 16 and thereafter is transmitted from an antenna 18.
In a receiver 20, a signal received by an antenna 22 is applied to a receiving section 24 and the spread spectrum signal is extracted from the received signal. Then, the extracted spread spectrum signal is correlated with the spread code by a correlator 26 whereby the spread spectrum signal is demodulated. Further, the transmitted data (correlated spread spectrum signal) is demodulated (by, e.g., PCM demodulation) by a demodulator 28 to provide demodulated data.
FIGS. 3a and 3b show examples of construction of receiving sections 24 of a prior art spread spectrum communication system. FIG. 3a shows a receiving section 24 in which a received signal is mixed directly with a local oscillation signal and thereafter a carrier component is removed from the mixed signal to obtain a spread spectrum signal. A signal P(t).multidot.sin 2.pi. fot (where P(t) represents a spread spectrum signal and fo represents frequency of the received signal (i.e., transmission frequency)) received by the antenna 22 is mixed with a local oscillation signal sin 2.pi. fLt (where fL represents local oscillation frequency) in a mixer 30 to provide a converted signal SM. This converted signal SM is filtered through a low-pass filter 32 to provide a signal SM' which contains a spread spectrum signal component.
FIG. 3b shows a receiving section 24 in which a received signal is once converted to an intermediate frequency and thereafter is mixed with a local oscillation signal to remove a carrier component and thereby provide a spread spectrum signal. A received signal P(t).multidot.sin 2.pi. fo't is mixed with a local oscillation signal sin 2.pi. fL't ( where fL' represents a local oscillation frequency) to produce an intermediate frequency signal P(t).multidot.sin 2.pi. fot (where fO represents intermediate frequency) in a mixer 34. This intermediate frequency is mixed with a local oscillation frequency signal sin 2.pi. fLt (where fL represents local oscillation frequency) in a mixer 30 to produce a converted signal SM. The converted signal SM is filtered through a low-pass filter 32 to provide a singal SM' containing a spread spectrum signal component.
In FIGS. 3a and 3b, the converted signal SM is expressed by the equation: ##EQU1##
This converted signal SM is applied to the low-pass filter 32 to have the fo+fL component removed and the following signal SM' thereby is obtained: EQU SM'=0.5P(t).multidot.cos 2.pi.(fo-fL)t (2)
In the past, the local oscillation frequency fL was set at fL=fo, i.e., at the same value as the transmission frequency fo. The equation (2) therefore becomes EQU SM'=0.5.multidot.P(t) (3)
and the spread spectrum signal P(t) thereby is obtained.
As described above, the local oscillation frequency fL was set at fL=fo in the prior art systems. It has been found, however, that it is actually difficult to achieve complete coincidence between the transmission frequency fo and the local oscillation frequency fL because there is variation in both of these frequencies fo and fL. For this reason, a difference component .DELTA.f=fL-fo is actually produced, so that the signal SM' becomes EQU SM'=0.5.multidot.P(t).multidot.cos 2.pi..DELTA.ft
In the spread spectrum communication, clock frequency fC of a spread code is generally set above several MHz with a resulting relation .DELTA.f&lt;fC. Accordingly, as shown in FIG. 4, the signal SM' becomes a waveform resulting from modulating the spread spectrum signal P(t) with a signal cos 2.pi..DELTA. ft and, as a result, the phase information of the original spread spectrum signal P(t) is impaired. This makes it impossible to demodulate both the spread spectrum signal and transmitted data (correlated spread spectrum signal) (FIG. 2). For preventing such impairing of the phase information of the original spread spectrum signal P(t), a phase locked loop circuit for enabling the local oscillation frequency fL to coincide exactly with the received signal frequency fo is required and the circuit construction therefore becomes complicated.
It is an object of the invention to provide a spread spectrum communication system capable of preventing a spread spectrum signal from being impaired due to variation in transmission frequency and local oscillation frequency without requiring a complicated circuit construction.